When 12.0.2 came out, we immediately tried the new release and also set up a new Lab server with this version (in our case running in a container). For a customer we also set up a dev environment using our application platform Aveedo where we are creating a brand new CRM system. Very soon we faced the problem, that pages were not loaded properly (404 error) at random occurrence. A normal reload mostly fixed the problem and the page was loaded - but not at a 100% rate. We never had this problem with prior versions of Domino.
After some testing with various XPages memory settings etc. I opened a case, and it turned out: there is a problem in the com.ibm.xsp.core_188.8.131.5221101-2131 plugin lib, which means in the XPages core.
HCL support immediately forwarded this to the dev team and they were able to create a testfix for us which solved the problem. The fix is addressed for FP1 of 12.0.2.